Final answer:
When a body of magma becomes more mafic, its viscosity will decrease as mafic magmas are hotter and contain less silica than felsic magmas, which results in a more fluid and less resistant flow.
Step-by-step explanation:
If a body of magma becomes more mafic, meaning it is richer in magnesium (Mg), calcium (Ca), and iron (Fe), its viscosity will decrease. Viscosity is the measure of a fluid's resistance to flow. Mafic magmas, which have a higher content of these elements, are hotter and typically have less silica (SiO2) than their felsic counterparts. Silica is the compound that contributes to higher viscosity in magmas. Therefore, as the amount of silica decreases with the increase in mafic minerals, the viscosity is reduced. This is because the mafic melts are generally hotter and the increased temperature leads to lower viscosity.
In more detail, viscosity is influenced by the chemical composition, temperature, and gas content of the magma. Higher temperatures and lower silica content in mafic magmas result in less polymerization of melt, making it more fluid and thus, less viscous. Given that viscosity plays a significant role in dictating the style of volcanic eruptions, understanding this property is essential in the study of volcanology.